There is a technique for modulating a refractive index by an electro-optic effect so as to switch an optical path. A device for obtaining a lens effect by an electro-optic effect disclosed in Patent Document 1 (Japanese Unexamined Patent Application No. Hei 1(1989)-230017) will be described below with reference to FIGS. 13 and 14.
The device of FIG. 13 has a configuration that an electro-optic medium (crystal material portion) 301 is placed between electrodes 302 and a voltage applied from a voltage power supply 303 is changed so that a change in refractive index is caused. Herein, the lens effect is obtained by forming each of the electrodes 302 into a shape capable of obtaining the lens effect, as shown in FIG. 13. The light condensing performance of the device includes a light condensing property in one direction, and the device functions as a so-called focal point variable cylindrical lens.
In addition, in FIG. 14, devices 314 and 316 each of which is the same as the above-described device are connected to each other via a half-wavelength plate 315. Herein, the half-wavelength plate rotates a deflection direction by 90 degrees, so that the device 314 and the device 316 provide the same effects in terms of modulation in the refractive index that is caused by the electro-optic effect.
In these connected devices, a light convergence effect in two directions, x and z in FIG. 14, can be obtained for the above-described reasons.
In the device having the electro-optic effect of Patent Document 1, each of the electrodes 302 is formed into a shape having a curved surface in accordance with the direction in which the optical path is desired to be switched, in order to obtain the lens effect.
In this configuration, one measure to effectively obtain the change in the refractive index that is caused by the electro-optic effect is to reduce the thickness of the entire crystal material portion 301 through which light passes in an optical path switching device by utilizing the property that the thinner the crystal material portion 301 is, the greater the change in refractive index.
In some cases, however, only a reduction in the entire thickness does not effectively lower a driving voltage. In the case where, for example, the crystal material portion 301 has a uniform thickness relative to the direction in which a ray bundle propagates, as in the device shown in FIG. 13, and when diverging light enters, the diameter of the ray bundle becomes maximum at an emission end of the device. Therefore, the ratio of light passing through the excessively thick crystal material portion 301 to light passing through the device becomes high on the light source side from the emission end. As a result, the electrical field intensity is relatively reduced for the same voltage, and an appropriate electro-optic effect for the light that passes through the device cannot be obtained.
In addition, though in the case where a great number of refractive regions in lens form are sequentially aligned in a direction in which the ray bundle propagates, the entire device has a great lens effect. However, in this case as well, the device has a shortcoming in that the greater the number of aligned refractive regions in lens form, the longer and larger the device becomes.
Taking the above into consideration, it is conceivable to form an optical path switching device where solely the thickness of the crystal material portion 301 is changed in a simple manner, as much as possible in accordance with a change in the ray bundle of the diverging light, so that the driving voltage for obtaining the same refractive effects in the refractive region having the same area approaches the minimum; however, the following problem arises in the case where this is simply implemented.
First, the crystal material portion 301 is partially made very thin; therefore, it becomes difficult to form a position reference for the assembly of the optical path switching device itself. In addition, the formation of the position reference on a plane that is not parallel to this optical path easily makes the position reference unclear. Furthermore, in some cases, the thickness of this crystal material portion becomes extremely thin; therefore, the electro-optic effect of the crystal material portion, i.e., an important portion of the optical path switching device, is negatively affected to a great extent, in such a manner that the crystal material portion is warped at the time of manufacture of the device or the crystal material portion is warped due to a change in the temperature after being made to adhere to a portion having a different linear expansion coefficient.
Furthermore, in the case where the above-described change in the thickness is achieved as a result of connection of crystal material portions (314 to 316) having different thicknesses, as shown in FIG. 14, the thinner the crystal material portions are made, the higher precision is required in a process for adjusting the optical path, and the manufacturing process itself becomes complicated when trying to solve the problem of reflection that is caused in the discontinuous structure, in such a manner that it becomes necessary to polish the respective surfaces to be connected as entrance and emission surfaces before the connection.
Here, in addition to the above, in the case where the thickness of the crystal material portion 301 is reduced in accordance with the ray bundle and the aperture is restricted at the emission end, the probability increases that the light that has failed to completely pass through the crystal material portion 301 is strongly reflected from the interface on the top and on the bottom of this crystal material portion 301. This reflected light mingles with the light emitted from the optical path switching device and, thereby, becomes stray light within the optical system where this device is used. Therefore, reduction of such light needs to be considered.